Processes that control the dissolution of loess sediments and contribution of arsenic release in the Chaco-Pampean plain, Argentina

Groundwaters of the Chaco-Pampean Plain in Argentina are contaminated by arsenic released from weathered volcaniclastic loess sediments. To better understand As release, the dissolution of three loessial materials (loess, volcanic glass shards (VGS) and volcanic ash) that make up the sediments was studied at pH 2-12 and 10-50 degrees C using flow-through experiments. The variation in the output concentrations was reproduced by 1D reactive transport simulations, enabling us to infer that the reactivity of the phases controlling the overall dissolution of the loessial materials varies as a function of pH. Oligoclase dissolves faster than K-feldspar and the Al-Si-amorphous phase at acid pH whereas the K-feldspar and Al-Si-amorphous phases dissolve more rapidly at basic pH. The steady-state output Si concentrations attributed to the main dissolving phases at different pH were used to calculate the dissolution rates of the loessial materials, yielding faster rates at acid pH than at the circumneutral to basic pH. The variation in the dissolution rate (mol m(-2) s(-1)) of the Al-Si-material making up the loess sediments in the shallow aquifers in the Chaco-Pampean Plain as a function of pH and temperature is expressed as R-diss = 10(-2.25).a(H+)(0.20 +/- 0.01) . e54/RT + H + 10(-8.51).e23.1/RT Simulations indicate that an initially high release of cations is due to rapid dissolution of fine particles and cation exchange between the exchangeable cations (Na, Ca, K and Mg) of the Al-Si-amorphous phase and those of the electrolyte solution. At steady state, dissolution of the coarse fraction was responsible for the cations release. The variation in the output As concentrations was simulated by considering two arsenic sources: (i) the arsenic from the As-rich fluorapatite and Fe-(hydr)oxides (e.g. ferrihydrite) that dissolve at acid pH and desorb As at basic pH, releasing arsenic into the solution, and (ii) Al-Si phases (e.g. volcanic glass), the dissolution of which releases the arsenic from the solid structure into the solution at any pH. At the start of the experiments, fluorapatite, ferrihydrite, glass an aluminosilicates contributed arsenic to the solution. At steady state, As was mainly released from the glass and the aluminosilicates, and the rate of As release from the loessial materials that constitute the Chaco-Pampean loess sediments was calculated using the aqueous As/Si ratios ((9.2 +/- 8.8).10(-5) and (7.4 +/- 2.2).10(-5) at acidic and basic pH, respectively).